Conventionally, fluid storage devices that store fluids and detect the amounts of the fluids therein are known. For example, Patent Document 1 (identified below) discloses a carbonization furnace for storing a powder (which is a kind of fluid), a stirring tank for agitating the powder, and a detector for detecting the powder surface of the powder.
The detector has a plate-like shape and is partially fixed to the carbonization furnace. In order to control the supply amount of the raw material powder and the discharge amount of the product powder in the carbonization furnace, the detector detects the powder surface by detecting the force generated on the powder during stirring.
Patent Document 1: Japanese Patent Application Laid-Open No. 2009-139207.
In the design disclosed in Patent Document 1, external vibrations including vibrations of the stirring tank may propagate to the detector via the carbonization furnace in some cases. Further, the detector disclosed in Patent Document 1 may cause bending displacement of bending in the thickness direction of the detector in some cases due to its plate-like shape.
Therefore, the conventional detector in Patent Document 1 has the problem that, even if the detector is not brought into contact with the powder, an electrical signal may be generated in the detector due to the bending displacement, and thereby leading to false detection.